1. Field of the Invention
The present invention relates to a strand comprising a multiplicity of glass fibers having a non-circular cross-section such as oval, elliptic, cocoon-shaped or a polygonal cross-section, as well as a method of producing such a strand.
2. Description of the Prior Art
In general, glass fibers are formed by spinning a multiplicity of filaments of molten glass from a bushing having numerous nozzle ports of a circular cross-section, gathering the filaments and taking up these filaments as a strand. Thus, each filament of the glass fiber has a circular cross-section. The glass fibers are used mainly as reinforcers mixed with a thermosetting resin or a thermoplastic resin. In general, a treating agent is applied to the surface of the glass fiber so as to produce a bonding force at the boundary surface between the glass and the resin.
In recent years, there is an increasing demand for higher strength of composite materials containing a resin and glass fibers. In fact, long glass fibers which are used in filament winding, pultrusion, roving cloth, electric insulating cloth and so forth are required to be formed into a strand having reduced thickness.
The present inventors have conducted an intense study to cope with such a demand and have found that enhancement of the bonding strength between the glass fibers and the resin is an important factor for attaining a high strength of the composite material. The inventors have found also that higher bonding strength is obtainable when the glass fibers have non-circular cross-sections such as oval, elliptic, cocoon-shaped or polygonal cross-sectional shape, as compared with the case where the glass fibers have a circular cross-section. This may be attributable to a fact that the glass fibers having non-circular cross-sections have specific surface areas greater than those of the glass fibers of a circular cross-section, so that they produce a greater bonding strength and, hence, a greater reinforcement effect. It has been confirmed also that, when the glass fibers have such a cross-section that the outer periphery is recessed inwardly, adjacent fibers are received by recesses of the adjacent glass fibers so as to resist any force which acts to separate the adjacent fibers from each other. The inventors have found also that, when the glass fibers have a comparatively flat cross-section such as an elliptic or cocoon-shaped form, the flat glass fibers are superposed so as to realize thin strands of the glass fibers. Unfortunately, however, it is very difficult to form glass fibers having non-circular cross-section. Namely, since the molten glass coming out the nozzle port has a low viscosity and, hence, a large surface tension at high temperature, the molten glass filament from the nozzle tends to have a circular cross-section even if each nozzle port has a non-circular cross-section. It is, therefore, impossible to obtain glass fibers having desired non-circular cross-sections.